1. Field of the Invention
The invention relates to a support body having an adsorber material for adsorbing nitrogen oxides in an exhaust gas that is generated in at least one combustion chamber of an internal-combustion engine. Support bodies of this type are preferably used in exhaust systems of mobile vehicles. Furthermore, the invention relates to an exhaust system that has a support body of this type and to a method of operating the exhaust system.
While in the event of complete combustion of a fuel composed only of carbon and hydrogen only the harmless substances of carbon dioxide (CO2) and water (H2O) are formed, exhaust gases from internal-combustion engines contain additional products resulting from incomplete oxidation, such as for example hydrogen (H2) and carbon monoxide (CO) or partially burnt or unburnt hydrocarbons (HmCn). Moreover, the exhaust gas contains the oxidation products of nitrogen NO and NO2. Since nitrogen oxides increase the formation of ozone close to the ground even in very small quantities (ppb range), emission of nitrogen oxides must be avoided.
In addition to the preventive measures aimed at reducing the nitrogen oxide concentration with the aid of a suitable engine management system, it is known to subject the exhaust gas that is generated to a cleaning procedure. To do this, by way of example a three-way catalytic converter is used, which removes the three most important pollutants (carbon monoxide, hydrocarbon, nitrogen oxide) from the exhaust gases. In this case, the carbon monoxides and the unburnt remainders of hydrocarbon are oxidized by the nitrogen oxide and oxygen to form carbon dioxide and water. The nitrogen oxides are reduced to form nitrogen. Although catalytic converters of this type allow these reactions to proceed quickly, they do not achieve complete reaction. One reason for this is that during certain operating states of the internal-combustion engine the residence time of the exhaust gas in the catalytic converter is so short that the reactions described above cannot take place completely. Effective reduction of the nitrogen oxides is only possible if a certain quantity of hydrogen or carbon monoxide is present in the exhaust gas while significant concentrations of oxygen are unacceptable. Therefore, the internal-combustion engine would preferably have to be operated in rich-burn mode (lambda<1), but this is undesirable in view of fuel consumption.
U.S. Pat. No. 5,795,553 discloses a support substrate with a coating that allows adsorption of nitrogen oxides in an oxygen-rich environment. This substrate desorbs the stored nitrogen oxide when a sufficient quantity of carbon monoxide and hydrocarbon is present in the exhaust gas. For this purpose, the coating has chemical compounds of an alkali metal with copper and hydrous zirconium oxide. The alkali metal used is preferably sodium or potassium. The support substrate allows nitrogen oxides to be stored when there is excess oxygen and ensures that the nitrogen oxides are desorbed when catalytic conversion using a catalytic converter is possible.
Furthermore, Published, Non-Prosecuted German Patent Application DE 196 36 041 A1 discloses a support substrate in honeycomb form with an adsorption layer for storing nitrogen oxides contained in the exhaust gas. The support substrate with an adsorption layer stores the nitrogen oxides when certain operating conditions prevail in the exhaust system, such as for example net oxidizing conditions (λ>1 and T>150° C.), and releases these nitrogen oxides again when operating conditions change, in particular when there is a stoichiometric air-fuel ratio and elevated temperatures. To convert the temporarily stored nitrogen oxides, a downstream catalytic converter is proposed which, at temperatures of over 230° C., converts at least 50% of the nitrogen monoxides contained in the exhaust gas into nitrogen dioxides.
When configuring the storage capacity of a support substrate of this type, the light-off performance of a downstream catalytic converter should also be taken into account. The use of relatively large-volume nitrogen oxide stores leads to very large amounts of thermal energy being extracted from the exhaust gas in the cold-starting phase of the internal-combustion engine. This leads to delayed heating of the catalytic converter, which only begins catalytic conversion of the pollutants in the exhaust gas once a temperature of approximately 250° C. is reached. If the storage capacity of the nitrogen oxide store is too low, the result may be emissions of nitrogen oxide and therefore pollution of the environment.